KFS-miniatures
tamiya's acrlisc are not the best for this technique- they are not an ordinary acrylisc and due ther excellent adhsion and really hard coat ther are really hard to 'damage'.
as a finial coat simple water based acrylics, like Lifecolor or Vallejo, are much better.
if after all for a final layer were used tamiyas or gunzes, the best liquid for wetting the surface is microscale's SOL decal fluid. in fact it is strongly thinned cleaner for acrylics. it's a bit caustic for the paint surface and softing it well, but not enough to damage the base coat protected with clear warnish.
With all due respect to you, KFS (and not wanting to hijack your thread, Chuck), I have to refute this assertion. In all seriousness, I don't think that you quite understand the technique? You're making the incorrect assumption that the key to the technique is about how you dissolve/attack
the paint--which is not the way the technique works.
I use Tamiya acrylics exclusively for the hair spray technique, and they are an excellent choice. In fact the hair spray technique has very little to do with the actual BRAND of paint, and really has everything to do with the TYPE of paint and both the type and brand of hair spray, as well as the way you apply it.
Here's how it works. Hair Spray is water-soluble. Therefore, the technique involves the application of paint over a water-soluble coat of hair spray, which--when attacked by water--will dissolve under the paint, carrying away the paint in varying degrees depending on several factors.
First is the type of paint coat that you first put the hair spray over. The glossier the paint, the larger "chunks" of paint that will usually come off--and the harder it can be to control if you're hasty. Enamel paints are perfect for the first coat, but a coat of gloss lacquer over any acrylic will work. In fact, in THAT case, Tamiya acrylics are an excellent choice because of their relatively hardier resistance to "fogging" or degradation to a lacquer coat.
Second, the type and depth of the hair spray coat is important. I go for Aqua Net hair spray precisely because it's a cheap, nasty hair spray. (I know it because I used it for my own "Tower of Power" teased-up big hair in the 80's; think Axl Rose in the "Welcome to the Jungle" but 10x higher! lol) Aqua Net is easy to work with and "cheap" means a high water content without a lot of other chemicals designed to conditioner your hair at the same time, blah, blah--which translates into "harder to work with". The heavier the coat you put down, the more it will react to the water and more easily it will be to remove. Now, don't "drown: the model in it--Aqua Net sprays pretty heavy, so one good, covering coat suffices for me.
How long you wait after you spray both the hair spray AND the paint down is a factor too. Obviously, the sooner you start the removal process, the easier it is to remove the hair spray coat, and the easier it will be to remove the paint if the paint itself is not dried completely. I know I know--I said that attacking the paint is not the key, but in this case, the barely-dried paint will let more water through to dissolve the hair spray and in that case lead to greater dissolution of the hair spray, ergo, more overcoat of paint will come off as well. I give the hair spray about an hour to dry, and I start in on the paint coat after about 10-15 minutes after I spray it. Longer (maybe an hour?) if I want a more restrained look; i.e., less paint removal.
You should also have planned in advance to have a few "bare areas" where you mask small chips of masking tape that you can remove to use as "entry points" for the next step--the application of water. If you cover the entire model with paint, it's going to be harder to get water UNDER the paint without some sort of breach of that coat.
Next is the way you remove it. You should use warm water--NOT a decal solution! You'll just be wasting expensive decal solution for nothing and risking attacking the paint itself--which is NOT the point of the technique. Wet the area, let it sit for 5-10 minutes, and then go to town. Use anything you want to scrape away--stiff brushes, toothpicks, metal implements, sanding sticks. I've used all the above for different effects and depth of removal.
Lastly, check out my Impala article in the new Finescale. It details the whole technique there.
Have fun and good luck!